In repairing living tissues, sutures or clips are routinely used to close defects, join planes of tissues or to join bodily tubes together (anastomoses).
This involves the placing of materials in the body which cause some damage to the tissues involved, but hold those tissues in apposition while the body's own healing processes effect a more permanent join. The damage that various joining materials cause varies but even careful placement of microsutures in the smallest of bodily tubes during an anastomosis produces a fibrous tissue reaction around each of the suture materials left in situ.
Joins, however made, take time, and those joins made by placing individual sutures in tubular joins are the most time consuming. Sewing in a ring of sutures to effect such a join inside the body may demand a large incision to obtain the access required to effect enough surgical freedom to manipulate the equipment and instruments required. Microsuturing requires considerable skill.
Arteries and Other Tubes
Fluids, and materials suspended within them, can travel along the body's patent tubes. Arteries carry blood from the heart to other organs and tissues in the body. They have 3 layers, an inner specialised mucosa (termed the intima), a thicker, middle, muscular and structural layer which contains collagen and elastin connective proteins (the media), and an outside layer which is a scaffold with fibrous tissue, blood vessels and nerves all supplying the functions of the artery (the adventitia). The inner volume of the artery is the lumen.
For tubes such as arteries to function in transporting blood at high pressure, they need to be strong. They are actually active in transporting a pressure wave of blood by expanding and relaxing (systole and diastole) as the bolus of blood passes. Joining such active tubes requires such physiological activity as promoting blood flow to be considered and the design of methods of anastomosis that will allow the activity to continue after the join.
Injuries to an artery are potentially very serious for an animal or human, as blood flowing through the artery is at high pressure and blood loss can be rapid. If the intima layer is damaged, then the middle, structural layer, the media, is exposed to blood. This triggers an important repair mechanism which acts to seal the wound and prevent further bleeding by the formation of blood clots on the wound, caused by blood coming into contact with the exposed collagen of the media.
Although microsuturing is the standard clinical repair technique for a severed artery, it has several disadvantages. A high skill level is required to make between 6 and 12 separate sutures to repair the artery. The sutures remain in the body acting as a site for fibrous tissue to form due to foreign body reaction, and this fibrous tissue is a point of weakness in the artery even after it is deemed to have healed. Although suturing does not produce a fluid-tight seal, surgeons usually rely on blood clotting triggered by the mechanism described above to seal the vessel soon after the repair is complete.
A number of laser-assisted welding techniques have been explored in order to find a more convenient technique which does not lead to so much scarring. These almost always need stay sutures (sutures used to join the vessels before laser treatment, which may or may not be removed subsequently) for a successful outcome. In this case the two vessel ends are held together to allow stay sutures to be inserted and then a laser is used to heat the tissue at the join so that proteins at the site are coagulated and bond together. Lasers such as the infra-red holmium-doped YAG and carbon dioxide lasers have been used because these produce wavelengths which are strongly absorbed by water in the tissue. Alternatively a dye solution may be applied to the tissue to enhance light absorption at a suitable laser wavelength. In any case, it is crucial that the intima layers of the 2 ends are in continuity, to avoid a blockage or a clot and to promote smooth laminar flow in the repaired vessel. This is difficult to achieve in thick-walled vessels where the laser energy may not be absorbed through all three layers of the vessel to form a strong weld with a smooth intima layer.
Some protein glues have been used to repair blood vessels, such as fibrin (which triggers a blood clotting reaction to effect a tissue join). A possible disadvantage of such a glue is the potential to be associated with blood clotting within the vessel, partially or wholly obstructing it.
Laser-activated fluid albumin solder has also been used, but the solder has required stay sutures to achieve sufficient repair strength for arteries which carry blood at high pressure. Fluid glues and solders tend to run between the tissue ends, risking blockage of the inner lumen, and are difficult to control and position accurately on the tissue repair. To attain a seal, they have been applied circumferentially around the join, which is then circumferentially welded. These joins later show thick scarring which can cause stricture or blockage of the vessel or tube.
There is also a lack of precision in such techniques, because of differences in the glue or fluid solder consistency, variations in the type of applicator device used to apply the glue or fluid solder, and the pressure needed to form a join.
A major drawback with current fluid solders is that they rapidly deteriorate and change composition when introduced into moist environments.
Similarly, existing solid solders must be kept dry when introduced to moist arteries, to prevent them from absorbing moisture, weakening their internal bonding and losing strength, even though this occurs more slowly than for fluid solders.
The repair of other bodily tubes is similar in is concept. Since the structure of each tube is specialised to its function and the nature of its contents there must be careful choice of the method of tube repair so that it will not interfere with the tube function, and in particular with maintaining the inner lumen of the tube.
Peripheral Nerves
The electrical signals that control the body's organs and transmit information back and forth to the central nervous system (CNS) travel along peripheral nerves.
A peripheral nerve has an outer membrane consisting of connective tissue such as collagen. This membrane (epineurium) protects and holds separate bundles of nerves or fascicles together. The fascicles group together nerve axons supplying a specific region of the body and are bounded by perineurium membranes. Each axon is supported by a Schwann cell within the fascicle. Nerve metabolism is sustained by the vascular system from both outside and within the nerve.
When a peripheral nerve is cut all axons distal (further from the spine) to the wound change their properties. Even when the nerve is reconnected, these axons continue to degenerate distally. The Schwann cells which normally wrap themselves around the axons as insulation, guide regenerating axons. Joining nerves as accurately as possible by lining up corresponding fascicles enables the enclosed axons to more efficiently regenerate.
Peripheral nerves can have diameters ranging from approximately 1 cm to approximately 50 micrometers.
Operating on nerves and other tissues of small dimensions has been facilitated by using magnification and special microsurgical equipment. Accurate nerve repairs need to be effected at the fascicular level ensuring that regeneration is along the correct bundle leading to the original area those axons supplied.
The current technique of peripheral nerve repair uses microsuturing. This technique requires a dedicated trained surgeon as microsuturing of just one of the many fascicles with three or more microsutures (using say a 70 micron diameter needle and 30 micron thread) can take very long operating times. There is the prospect of added damage to the inner axons due to sutures penetrating the thin perineurial sheath. The use of sutures results in some scarring of the repair due to foreign body reaction. Excessive scarring impairs nerve function and may be associated with painful neuromas. There is also evidence that in the long term, scar tissue formation and scar maturation can impair the joined nerve.
Work has been performed on the use of lasers alone in effecting nerve joins. To date the welds have typically been made using infrared lasers such as carbon dioxide lasers which rely on water absorption for energy-transfer. Tissue preparation before welding relies on overlapping the nerve membranes. One of the problems of laser welding has been the fact that the intact axonal tissue is under pressure within the fascicle, so that when it is cut the axons extrude. Laser treatment can thus lead to denaturation of the axon material leading to scarring and proliferation of fibrous tissue.
Laser-activated protein solders have also been tried, as described for the artery and blood vessel case above. Again because of difficulties in controlling fluid solders, and the weakness of the resulting bonds in a moist environment, these repairs are usually too weak without the addition of stay sutures. This complicates the surgical technique and leads to additional scarring and foreign body reaction.
The bonds formed to date as described in the prior art using laser welding have typically lacked strength and thus microsuturing has been used in addition to welding to strengthen these joins.
Solutions to at least some of these problems are taught in WO96/22054. The present invention relates to alternative solutions.